South Korean director Bong Joon-ho specializes in such genre changeups with a commanding sense of spin and velocity. You think you see clownish convention, quirky indulgence or grim self-righteousness coming out of the corner of your eye, but being fooled rarely feels so good.

Dumped formaldehyde turns Seoul’s Han River into a moat for a monster, which strikes in broad daylight. Bong’s coiled, scaly creature treats overpass girders like a gymnast would parallel bars, but has better things to do than exist as a cool visual effect. Watch how the beast skitters nervously, displaying panic and survival instinct while it turns a picturesque park into a killing floor during a mercenary rendition of “Jurassic Park’s” brachiosaurus run.

After a deadbeat dad (Song Kang-ho) gets a phone call from his daughter — snatched by the behemoth and believed dead by the authorities — his fractured family attempts a rescue alone.

As in his equally masterful “Memories of Murder,” Bong audaciously poked around for nervous laughter in personal and political failings — estrangement and competitiveness fuels the familial crises, while the government covers its ass with chemicals and calls it containment. It all led to an unexpectedly bittersweet ending of rage and redemption found within the copious collateral damage caused by the family, creature and country.